1. Field of the Invention
The present invention relates to burner control and, more particularly, to a thermoelectric sensor and to a burner apparatus incorporating the thermoelectric-sensor.
Applicants are primarily interested in fully premixed air/fuel gas burner apparatus. By a fully premixed air/fuel gas burner apparatus Applicants mean one in which the fuel gas is mixed, prior to combustion, with all the air required for complete combustion, the combustion air being supplied by mechanical means and hereinafter referred to as "fan-means".
2. Description of the Related Art
A fully premixed air/fuel gas burner apparatus may employ a flamestrip which may be porous or have a plurality of burner ports or apertures therethrough, for example a ceramic flamestrip, to support the flame. The flamestrip may be a discrete part of the burner or, alternatively, may be integral with one or more other parts of the burner. In either case it is possible that the flame may be caused to burn very close to the flamestrip, for example when the flowrate of air in relation to the flowrate of fuel gas has, for whatever reason, decreased to about 10% in excess of that theoretically necessary for complete combustion, corresponding to an air/fuel gas mixture aeration of 110%. This can cause a rapid increase in burner temperature, particularly at low rates of heat output per unit of total flameport area (otherwise referred to as low port loadings). If this situation were allowed to persist, progressive overheating might occur and result in the flamefront entering the ports of the flamestrip and igniting the air/fuel gas mixture inside the burner. This dangerous condition is termed `lightback`.
If an air/fuel gas mixture of high aeration, for example 160%, is supplied to the flamestrip, particularly at high port loadings, the velocity of the air/fuel gas mixture through the ports in the strip may become greater than the speed at which the flame can burn at the ports. The flame would then burn away from the flamestrip--a condition referred to as "flame lift". If the speed of the mixture is sufficiently greater than the flame speed, the flame front will be pushed or blown away from the flamestrip completely and the flame will disappear.
Furthermore, in combustion equipment it is generally the case that the position of the flame front varies with the rate of heat output at fixed aeration, the flamefront moving away from the flamestrip as the rate of heat output increases.
It will therefore be apparent that the position of the flamefront in fully-premixed combustion varies according both to the aeration of the air/fuel gas mixture and to the rate of heat output. In a system where the combustion air is supplied by fan means, in order to achieve a stable flame, means of controlling the rate of air supply (and so, the aeration) should, desirably always, be used, and must be used if the heat output of the burner is to be varied appreciably. In such a system, aeration control is most advantageously of the `closed-loop` kind, comprising a variable-speed fan for supplying air, a modulating fuel gas valve, a means for measuring the air/fuel gas flowrate ratio and a control means to control the rates of air and fuel gas supply, so as to match these appropriately to each other by varying the fan speed and/or the fuel gas valve opening. The adoption of a `closed-loop` aeration control system allows the operation of an appliance to be largely independent of the combustion characteristics of the fuel gas supplied, and also allows compensation as necessary for variations in the performance of the fan means, in supply voltage, and in the flow resistance of the flue and/or heat exchanger.